If you are checking someone out, you are clearly both acting as members of the flight crew and should log it as required in Art 35(2).
1. Check Insurance / club / group / operator rules to see who can be the person providing the required "check". Be careful with regard to insurance requirements because in the event of you saying "this pilot was competent" and they do something that results in a claim, the insurance company may ask "what qualified you to make such an assessment?".
2. Both members can log the flight.
Insurance is now a regulatory requirement. Unless the insurance requirements are followed then the flight is uninsured and therefore illegal.
Following from that it is clear that two pilots are required under the regulations and the person providing the check is PIC and loggs that while the other pilot is a co-pilot and should log that time as co-pilot.
NO! As one or two have already pointed out there is no sign off required. As a non FI you are not qualified to sign off anything in a log book however; as a a pilot who is familiar with the type there is nothing to stop you familiarising another qualified pilot as part of an insurance requirement.
There must be a defined record of having given and received the checkout. Unless a certificate is going to be produced then the only alternative and the method I would recomend is that the person providing the checkout makes an appropriate note and signs in the logbook of the pilot receiving the checkout. That keeps the required information in a place where it can be found easily should it be required. It also enables the PIC to ensure that his/her co-pilot recorded the flight correctly.
Recording the flight (which could be more than 1 hour duration) as co-pilot not only meets the legal requirements but also ensures that there is no posibility of later confusion when the pilot presents their logbook to an examiner who will look for 1 hour of dual training prior to extending the validity fo their SEP classs rating.
The important things to remember is that;
The person being checked can not be PIC and no dual time can be logged.